Method for removing harmful components from a gaseous mixture

ABSTRACT

Compounds having the formula 
     
       
         A 2 B 3 O 6±d   
       
     
     wherein A is an alkaline-earth metal, an alkaline metal, a lanthanide, or a solid solution thereof, B is a transition metal, an element of group III, or a solid solution thereof, and d has a value between 0 and 1; a method for preparing the compounds; a method for producing composite materials on various matrices and thin or thick films deposited on various substrates which contain the compounds; their use; and a method for eliminating certain gases from a mixture that includes them by using the compounds.

BACKGROUND OF THE INVENTION

The present invention relates to a new class of compounds having agas-fixing activity, to a method for preparing said compounds, tomethods for producing composite materials on various matrices and thinor thick films deposited on various substrates and containing saidcompounds, and to their use, as well as to a method for eliminatingcertain gases from a mixture that comprises them by using saidcompounds.

Various classes of materials with gas-fixing capabilities are currentlyknown. They can be divided into two categories, depending on whether a)the fixing properties depend on actual chemical reactions, which entailthe decomposition of the fixing material or b) the fixing propertiesdepend on the adsorption characteristics at the physical surface of thefixer and, in general, on the size of the molecules to be fixed. Sometypical examples of type a) materials are compounds capable ofeliminating water vapor from a mixture of gases, for example calciumsulfate, phosphorus pentoxide, magnesium chloride, or carbon dioxidefrom a mixture of gases, for example sodium and potassium hydroxides andcalcium, strontium, and barium oxides. Classic examples of type b)fixing materials are materials having an activated surface, such asactivated charcoal or the various types of zeolite or some kinds ofclay.

The fixing properties of type a) materials are selective, in that acompound is capable of fixing a single type of gas. The range of usablespontaneous reactions is rather limited and does not include gases whichare highly harmful to the health and to the environment, such asnitrogen monoxide NO and carbon monoxide CO. Moreover, the involvedreactions may be irreversible, so that the fixer loses all activityafter a given utilization cycle.

On the other hand, type b) materials are not selective and fix gasmolecules according to their size, degree of polarity, and relativemolecular mass. These materials are unable to fix light molecules, suchas the combustion products that are most harmful to the health and tothe environment, such as mixtures of nitrogen oxides NO and NO₂ andcarbon oxides, particularly the monoxide CO.

SUMMARY OF THE INVENTION

A principal aim of the present invention is to eliminate the drawbacksof conventional materials having gas-fixing capabilities, withparticular interest for the removal of noxious components fromcombustion products and more generally from gas mixtures.

This aim as well as other objects which will become apparent from thefollowing detailed description of the invention are achieved by a classof compounds according to the invention, which is represented by acompound having the formula A₂B₃O_(6±d), wherein A is an alkaline-earthmetal, an alkaline metal, a lanthanide or a solid solution thereof; B isa transition metal, an element of group III, or a solid solutionthereof; and d has a value between 0 and 1.

Advantageously, A is chosen from the group constituted by barium,cesium, potassium, strontium, a lanthanide, or solid solutions thereof.

Conveniently, B is chosen from the group constituted by copper, nickel,manganese, iron, palladium, titanium, aluminum, gallium, zinc, cobalt,or solid solutions thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction profile of Ba₂Cu₃O_(O±d).

FIG. 2 shows the U-shaped tubes used for removing the harmful componentsfrom a gas mixture.

FIG. 3 shows the result of an NO concentration analysis by massspectrometry on two gas streams.

FIG. 4 shows the electrical resistance of a film produced according toprocedure E) as stated below and exposed to a stream of NO₂ (50%) andair at room temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of compounds according to the invention, wherein A is a solidsolution of the above-mentioned cations, are those having the formula(Ba_(2−x)Sr_(x))Cu₃O₆ produced with values of x up to 0.75.

Other examples of compounds according to the invention, wherein B is asolid solution of the above-mentioned cations, have the formula

Ba₂(Cu_(3−y)Pd_(y))O_(d) produced with y up to 0.33;

Ba₂(Cu_(3−y)Ni_(y))O_(d) produced with y up to 1.0.

A compound having the formula Ba₂Cu₃O_(5+x) has been identified andpartially described in the literature (see for example W. Wong-Ng and L.P. Cook, Powder Diffraction, 9 (1994), p. 280-289 and the referenceslisted therein). However, the researchers who preceded the inventors ofthe present invention did not realize that they were in the presence ofa new class of compounds having particular chemical activitycharacteristics.

In the compounds of the class to which this description relates, severalphenomena which are intermediate between the behavior observed in typea) and type b) fixing compounds have been observed for the first time.The fixed gas molecules in fact react with the fixing material, in thatthey are truly bonded to the structure of the solid in the form ofanions, as in type a) materials. However, the chemical reaction, withinwide limits in terms of fixed gas amount, does not produce thedecomposition of the fixing compound, the structural characteristicswhereof vary to a very limited extent and in any case continuously andreversibly as a function of the amount of gas removed from the reactionatmosphere, similarly to the behavior observed in type b) fixers.Moreover, the fixing properties of the compounds according to theinvention are not selective as in type a) materials and occur with arelatively wide variety of gases, such as gaseous halogens and oxides.

Compounds having the formula A₂B₃O^(6±d) according to the presentinvention can be prepared by direct reaction starting from mixturescontaining oxide, peroxide, and nitrate precursors. These compounds leadto a characteristic X-ray powder diffraction profile, a typical examplewhereof, related to a sample of Ba₂Cu₃O_(6±d), is shown in FIG. 1 (CuKaradiation was used). The spectrum in the figure can be indexed on thebasis of an orthorhombic cell with a=4.316(1), b=6.889(2), andc=11.442(3) Å; the cell parameters, however, undergo significantvariations as a function of d.

The formation temperatures of the compounds according to the presentinvention are typically within the range of 300 to 950° C. The optimumvalues of course vary as a function of the cations being used.

The method for preparing the compounds having the formula A₂B₃O_(6±d)according to the present invention comprises a stage for the heattreatment of mixtures of oxides, peroxides, or salts of the requiredcations in highly oxidizing conditions. For example, heating occurs in acontrolled atmosphere that contains only oxygen, nitrogen, and inertgases.

Of course, the higher the content of oxidizing compounds (for exampleperoxides or nitrates) in the initial mixture, the lower the partialpressure of oxygen in the atmosphere required to prepare the chosencompound. Vice versa, the higher the pressure of the oxygen in thereaction atmosphere, the lesser the role of the oxidizing component inthe mixture of precursors.

The compounds according to the present invention can be prepared aspolycrystalline aggregates, as components in composite materials havingvarious matrices, and in the form of thin or thick layers on varioussubstrates.

Advantageously, the compounds according to the present invention can beprepared starting from a mixture comprising one or more oxygenatedcompounds of an alkaline or alkaline-earth metal and one or more oxidesof transition metals or elements of group III.

Moreover, the compounds according to the present invention can beprepared starting from a mixture comprising one or more nitrates of analkaline-earth or alkaline metal and one or more nitrates of transitionmetals or of elements of group III.

The following procedures illustrate the methods for preparing thecompound A₂B₃O_(6±d).

The following examples of a preparation method for the compoundsaccording to the present invention are given only by way ofnon-limitative example.

In all the procedures presented hereafter, the cationic molar ratio forproducing the indicated solutions and mixtures is given. For the sake ofgreater precision, the corresponding weight ratios are specifiedhereafter for each procedure.

Procedure A

A1) A mixture of fine powders of barium peroxide and copper oxide in a2:3 molar ratio [0.704 grams of copper oxide (CuO) for every gram ofbarium peroxide (BaO₂)] is produced;

A2) The mixture is homogenized by milling with a mechanical mill ormanually with a mortar and pestle of agate or with another method fordry mixing or for mixing in the presence of appropriate liquids;

A3) The homogenized mixture is placed in an inert refractory container(alumina or the like) and is heated in a furnace under a stream ofoxygen and inert gas (the partial pressure of the oxygen is typically≧0.2 bar, 1 cc/sec) at 580÷650° C.

A4) The compound is kept at the same temperature for 12 hours.

A5) Steps A2, A3, and A4 are repeated until a compound is obtained whichprovides the X-ray diffraction pattern shown in FIG. 1, whichcharacterizes the Ba₂Cu₃O_(6±d) phase.

Procedure B

B1) A mixture of fine powders of barium nitrate and copper oxide in a2:3 molar ratio [0.457 grams of copper oxide (CuO) for every gram ofbarium nitrate (Ba(NO₃)₂)] is produced;

B2) Same as A2.

B3) Same as A3.

B4) The compound is kept at the same temperature until the solution ofNO₂ gas generated by the decomposition of the nitrate salts is depleted.At the end of the process, the compound is obtained which gives theX-ray diffraction pattern shown in FIG. 1, which characterizes theBa₂CU₃O_(6±d) phase.

B5) With this procedure, the dimensions of the resulting granules are inthe millimeter range and allow the use of single-crystalcharacterization techniques.

Procedure C

C1) A mixture of fine powders of barium nitrate and copper nitrate in a2:3 molar ratio [1.335 grams of hemipentahydrate copper nitrate(Cu(NO₃)₂+2.5₂O) or 1.077 grams of anhydrous copper nitrate (Cu(NO₃)₂)for every gram of barium nitrate] is produced;

C2) Same as A2.

C3) Same as A3.

C4) Same as B4.

C5) Same as B5.

Procedure D

D1) A mixture of fine powders of barium oxide and copper oxide in a 2:3molar ratio [0.778 grams of copper oxide (CuO) for every gram of bariumoxide (BaO)] is produced;

D2) Same as A2;

D3) The homogenized mixture is placed in an inert refractory container(alumina or the like) and is heated in a pressurized furnace with apartial oxygen pressure in excess of 1 bar at 580÷650° C.;

D4) Same as A4;

D5) Steps D2, D3, and D4 are repeated until a compound is obtained whichhas the X-ray diffraction pattern shown in FIG. 1, which characterizesthe Ba₂Cu₃O_(6±d) phase.

Procedure E

E1) A solution of barium nitrate and copper nitrate in a 2:3 molar ratiois prepared in distilled water up to the solubility limit for bariumnitrate [1.335 grams of hemipentahydrate copper nitrate (Cu(NO₃)₂+2.5₂O)or 1.077 grams of anhydrous copper nitrate (Cu(NO₃)₂) for every gram ofbarium nitrate];

E2) An inert, temperature-resistant, porous medium (for example neutralactivated Brockman alumina) is impregnated with the solution thusprepared;

E3) The water is eliminated with a drying treatment at 150° C. for 2hours.

E4) Same as B4, but the reflections of the porous substrate, ifcrystalline, are also found in the X-ray diffractogram.

E5) The final product is a composite material wherein the compoundBa₂Cu₃O₆±d fills the microcavities of the porous substrate.

Procedure F.

F1) Same as E1;

F2) The solution is used to wet the surface of a substrate oftemperature-resistant, non-porous, inert material, constituted bypolycrystalline Al₂O₃ with a relative density of 99.9% (other examplesof usable non-porous inert materials are quartz, porcelain, Inconel,oxidation-resistant alloys and metals, etcetera);

F3) The deposited water is quickly evaporated by electrical heating toapproximately 230° C.;

F4) Steps F2 and F3 are repeated until a thin deposition of nitrates ofthe desired thickness, for example approximately 10 μ, is obtained;

F5) Same as B4.

F6) The final product is a film of Ba₂Cu₃O_(6±d) having a presettablethickness.

In another aspect, the present invention relates to a method foreliminating certain gases from a gaseous mixture including them. Theability of the compounds according to the present invention to fixmolecules of various gases directly from the gaseous state has beenverified by direct measurements, such as thermogravimetry, analysis ofthe gases in the reaction atmosphere, or by indirect measurements, suchas Raman and infrared spectroscopy. The compounds according to thepresent invention are particularly adapted for fixing oxides andhalogens in the gaseous phase.

Examples of such gases are NO₂, CO₂, SO₂, NO, CO, F₂, and Cl₂.

The way of fixing the gas depends on the type of gas and on thetemperature and composition of the atmosphere in which the reactionoccurs.

The inventors of the present invention have observed for the first timein the compounds of the class to which the present invention relatesfixing phenomena which are intermediate between the behavior observed inconventional fixing compounds of types a) and b) described above. Thegas molecules are in fact fixed by reacting chemically with the fixingmaterial, in that they are bonded to the structure of the solid in theform of anions. However, the chemical reactions related to the fixingprocesses, although continuously modifying the composition of the fixer,do not cause, over a broad range in terms of amount of fixed gas, thedestruction of the structure of the fixing solid, the structuralparameters whereof vary to a very small extent and in any casecontinuously as a function of the amount of gas removed from thereaction atmosphere. If the process is continued beyond these limits,absorption can continue but it entails the destruction of the fixerstructure and the formation of oxides or of the salts corresponding tothe anions that form.

The gases fixed by means of the mechanisms that are active in the firststages of the process, regardless of their nature, enter the compoundsaccording to the invention in the form of oxyanions or halide ions. Thestructural characteristics of the fixing compounds vary continuouslyduring this first part of the process, in the same way in which thestructural characteristics of a solid solution vary as one of itscomponents varies. During this stage, the fixing process is reversiblefor many of the fixable gases; by varying the partial pressures and thetemperature it is possible to desorb them fully or partially. Gaseousoxides with a low oxidation state (NO and CO, for example) can bedesorbed in a higher oxidation state (NO₂ and CO₂, for example) if theatmosphere in which release occurs is sufficiently oxidizing. Once afirst limit concentration of oxyanions or halides, which depends on thetype of gas and on the temperature, has been reached, reversibility islost except in the case of nitrogen oxides. In this particular case,reversibility of the fixing process is complete, since the final productcorresponds to the initial material for the production of the compoundsat issue, as described in preparation procedure B) or C), depending onwhether the process occurs above or below the temperature at which thecopper nitrate decomposes to copper oxide.

The chemical reactions related to the fixing processes occur by virtueof the presence of an excess of oxygen in the structure. By way ofexample, in the case of the compound in which A=Ba and B=Cu, the oxygencomposition produced by the normal state of oxidation of the cations(Ba²⁺and Cu²⁺) would be 5 atoms per unit of the formula (Ba₂Cu₃O₅),whilst accurate determinations of the oxygen content in samples preparedaccording to the previously described procedures show an oxygen contentthat is typically in the range of 5.5 to 6.1 atoms per unit of theformula. Accordingly, the fixing processes do not require the additionof external oxygen; however, their yield is increased by the presence ofoxygen in the gas mixtures placed in contact with the fixing materials.

The method according to the present invention for eliminating certaingases from a gaseous mixture that includes them consists in placing thegas mixture in contact with a compound having the formula A₂B₃O_(6±d) inpure form or as a component of composite materials, in which A is analkaline-earth metal, an alkaline metal, a lanthanide, or a solidsolution thereof; B is a transition metal, an element of group III, or asolid solution thereof; and d has a value between 0 and 1, at atemperature between the melting temperature of the compound to be fixedand 650° C. Preferably, A is chosen from the group constituted bybarium, cesium, potassium, strontium, or solid solutions thereof, and Bis chosen from the group constituted by copper, nickel, manganese, iron,palladium, titanium, aluminum, gallium, zinc, cobalt, or solid solutionsthereof. The method is described in greater detail hereinafter with twoexamples referring to situations that produce different yields:

1) fixing of NO₂ at room temperature from an oxygen-containingatmosphere;

2) hot fixing of NO from an oxygen-free atmosphere.

The following examples of gas fixing methods using compounds accordingto the present invention are given merely by way of non-limitativeexample.

EXAMPLE 1 NO₂ Fixing

3 grams of Ba₂Cu₃O_(6±d) produced according to procedure B) are placedin a U-shaped tube wherein a stream of NO₂ (50%) and air is made to flow(90 cc/min flow, 20° C. temperature). The gas fixing activity is clearlyshown in FIG. 2, which shows a first U-shaped tube (1) and a secondU-shaped tube (2) which are parallel-connected with respect to thestream of NO₂ and air; a layer of Ba₂Cu₃O_(6±d) is present in the baseof tube (1), whilst a layer of white material (for example cotton) ispresent in the base of tube (2). It is observed that in tube (1) the gaschanges color from orange (represented by stippling) to colorless as aconsequence of its passage through the layer of Ba₂Cu₃O_(6±d), whilst intube (2) it remains of the same orange color after passing through thewhite layer. In these conditions, the fixing activity persists forapproximately 300 minutes. At the end of the process, a weight increasewas found which corresponded, within the measurement errors (5%), to thetotal conversion of the initial compound into copper and barium nitratesalts and to the fixing of 10 moles of gas per mole of fixer.

EXAMPLE 2 NO Fixing

FIG. 3 shows the result of an NO concentration analysis by massspectrometry on two gas streams (112 cc/min) originating from a commonsource of a mixture of He+0.4% NO, one of which passes through a reactorbrought to 350° C. which contains 1 gram of Ba₂Cu₃O_(6±d) compoundproduced according to procedure A). This measurement points outdifferent fixing processes with separate kinetics. The amount of gasfixed at saturation (2 hours) is 0.28 moles per mole of fixer.

Another aspect of the present invention relates to an electric sensorfor gas concentration, which comprises a compound having the formulaA₂B₃O_(6±d). The inventors have in fact found that for the compoundsaccording to the present invention, during the gas fixing process, thevalue of electrical resistivity increases in proportion to the amount ofgas that is incorporated. For example, the compound Ba₂Cu₃O_(6±d) is asemiconductor with typical values of electrical resistance at roomtemperature on the order of 10-100 ohm/cm. In the case of the fixing ofnitrogen or carbon oxides, this value is up to 4 orders of magnitudehigher. FIG. 4 plots the electrical resistance of a film producedaccording to procedure E) and exposed to a stream of NO₂ (50%) and airat room temperature.

Moreover, in another aspect the present invention relates to optical gasconcentration sensors which comprise a compound having the formulaA₂B₃O_(6±d).

The inventors of the present invention have found that the compoundshaving the formula A₂B₃ ₆O_(6±d) show considerable variations in theiroptical properties during the gas incorporation process. Thesevariations become apparent as variations in the intensities of thecharacteristic modes in infrared and Raman spectra, with the appearanceof new optical modes caused by the characteristic vibrations of theincorporated anions and with the appearance of characteristics whichcannot be ascribed to the initial material or to the guest anions, suchas for example a plurality of highly intense luminescence peaks whichappear as a consequence of the incorporation of small amounts of carbonoxides. In addition to the variations in the measured spectra,macroscopic changes in color are observed as a consequence of theincorporation of small amounts of carbon oxides. For example, the colorof the Ba₂Cu₃O_(6±d) changes from the initial dark blue to black, whilsta color change is observed towards greenish pale blue as a consequenceof the incorporation of large amounts of nitrogen oxides below 170° C.

What is claimed is:
 1. A method for removing harmful components in agaseous mixture comprising the step of contacting the gaseous mixture atroom temperature with a compound having the formula A₂B₃O_(6±d) whereinA is an alkaline-earth metal, an alkaline metal, a lanthanide, or asolid solution thereof, B is a transition metal, an element of groupIII, or a solid solution thereof, and d has a value between 0 and
 1. 2.The method of claim 1, wherein A is chosen from the group constituted bybarium, cesium, potassium, strontium, a lanthanide, or solid solutionsthereof.
 3. The method of claim 1, wherein B is chosen from the groupconstituted by copper, nickel, manganese, iron, palladium, titanium,aluminum, gallium, zinc, cobalt, or solid solutions thereof.
 4. Themethod of claim 1, wherein said compound has the formula Ba₂Cu₃O_(6±d).5. The method of claim 1, wherein said compound has the formula(Ba_(2−x)A_(x))Cu₃O_(6±d) wherein A is an alkaline metal, analkaline-earth metal, or a lanthanide.
 6. The method of claim 1, whereinsaid compound has the formula (Ba_(2−x)Sr_(x))Cu₃O_(6±d), 0<×<0.75. 7.The method of claim 1, wherein said compound has the formulaBa₂(Cu_(3−y)B_(y))O_(6±d), 0<y<1, wherein B is a transition metal or anelement of group III.
 8. The method of claim 2, wherein said compoundhas the formula Ba₂(Cu_(3−y)Ni_(y))O_(6±d), 0<y<1.
 9. The method ofclaim 1, wherein said compound has the formulaBa₂(Cu_(3−y)Pd_(y))O_(6±d), 0<y<0.33.
 10. The method of claim 1, whereinsaid components are selected from the group consisting of gaseoushalogens and oxides.
 11. The method of claim 1, wherein said componentsare chosen from the group constituted by NO, NO₂, CO₂, CO, SO₂, F₂, andCl₂.
 12. The method according to claim 11, comprising a subsequent stepfor the regeneration of said compound by heating to a temperaturebetween 550 and 750° C.
 13. The method according to claim 12, whereinsaid regeneration step occurs in an oxidizing atmosphere.
 14. The methodof claim 1 wherein the compound is in the form of a composite materialcomprising the compound.
 15. The method of claim 1, wherein the compoundis in the form of a film constituted by a material comprising thecompound.